1. Technical Field
The present disclosure relates to a transfer device and an image forming apparatus incorporating the transfer device, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers, having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data.
In electrophotographic image forming apparatuses, an electrostatic latent image, which is obtained by forming optical image data on an image carrier (e.g., a photoconductor) that is uniformly charged in advance, is rendered visible with toner from a development device. An image is formed on a recording medium by transferring the visible image directly or indirectly onto the recording medium (e.g., transfer sheet) via an intermediate transfer device and fixing the image thereon.
There are two types of transfer devices, a direct-transfer type and an intermediate transfer type. In an image forming apparatus containing the direct-transfer type transfer device, a toner image is formed on an image carrier (e.g., a drum-shaped or belt-shaped photoconductor) that then directly transfers the toner image onto a recording medium using a transfer member (e.g., a transfer roller).
Alternatively, the toner image is formed on the photoreceptor and then primary-transferred onto an intermediate transfer member (e.g., an intermediate transfer belt) serving as a second image carrier, after which the toner image on the intermediate transfer member is secondary-transferred onto a recording medium by the transfer member.
In above-described intermediate transfer device, when a high-asperity sheet, such as Japanese Paper, is used as the recording medium, a pattern of light and dark shading conforming to the surface condition of the recording medium may easily appear in an output image. This shading pattern appears because the toner is transferred poorly to concave portions on the surface of the recording medium, and as a result, the density of toner at the recessed portions is less than that at the convex portions. In order to solve this problem, instead of using a secondary bias composed only of a direct current voltage, a bias may be used in which a direct current voltage is superimposed on an alternating current voltage, thereby preventing the shading pattern from occurring.
Moreover, by varying the composition of the applied transfer bias voltage depending on the surface roughness and electrical resistivity (comparable to thickness) of the recording medium, deterioration in toner transfer efficiency due to too strong an electrical field generated by the transfer bias current and consequent uneven image density can be avoided. However, as the AC electrical field is strong, the discharge ability of the toner image and the recording medium become strong. If the AC electrical field is excessive, the toner image on the image carrier, charged to a predetermined polarity, is discharged, and the transfer efficiency may be reduced.
However, in a transfer device in which the toner image on the image carrier, charged to the predetermined polarity, is transferred onto the recording medium using the transfer electrical field, an electrostatic field may be formed between the recording medium and the image carrier, because the recording medium is charged by the transfer electrical field. Then, the recording medium cannot be separated from the image carrier, which causes the recording medium to be jammed.
In general, the image carrier curves sharply away from a recording medium transport path immediately downstream from a transfer zone in which the transfer electrical field is formed. Therefore, even if the recording medium is charged by the action of transfer, the rigidity of stiff recording media such as cardboard can overcome the electrostatic attraction between the image carrier and the recording medium. As a result, the recording medium goes straight without being attracted to the image carrier and bending with the curve, which prevents the sheet from jamming.
However, with pliant recording media having weak rigidity, such as thin paper, the electrostatic attraction between the recording medium and the image carrier may be stronger than the rigidity of the thin paper. Therefore, the recording medium tends to adhere to the image carrier and bend with the curve, thus deviating from the transport path and resulting in jamming.